8 things to know about the vernal equinox

How eggs, shamrocks and monuments tie into the first day of spring.

The vernal equinox is when the Northern Hemisphere ushers in spring.
(Photo: Alyona Naive Angel/Shutterstock)

The vernal equinox, also known as the spring equinox or the March equinox, occurs on March 20 this year, when the Northern Hemisphere says farewell to winter and ushers in spring. It's one of two days each year (the other is the autumnal equinox) when day and night are each about 12 hours long in all parts of the world.

While you may associate this time of year with blooming flowers and lighter jackets, the vernal equinox has a rich history of celebration dating back thousands of years. Here are eight facts you may not know about the start of spring.

1. It doesn't fall on the same date every year. The vernal equinox can take place on March 19, 20 or 21. Some folks assume that March 21 is the first day of spring, but that's not always true. In this century, the equinox has fallen on March 21 only twice, in 2003 and 2007.

The date changes for three reasons, according to Space.com. First, neither a year nor a season is an even number of days, and our calendar was constructed to give an approximation of how long it takes the Earth to make one full orbit around the sun. Second, the planet's orbit is changing, which causes the Earth's axis to point in different directions, which affects the time it takes for Earth to reach a 90-degree location (more on that in #2) in its orbit around the sun. And third, the gravitational pull from other planets can impact Earth's orbit, too.

The vernal equinox may occur on different dates in different time zones. For example, countries ahead of UTC (Universal Coordinated Time) may celebrate the March equinox a day later.

2. It occurs at a specific moment, not all day long. We may celebrate the equinox as a day, but it's actually just one moment. The equinox occurs when the sun crosses the celestial equator (an imaginary line above the Earth's equator), and the Earth is perpendicular (90 degrees) to the sun's rays.

3. It's one of only two days when this experiment will work. This shadow trick comes courtesy of TimeandDate.com:

You'll need a straight stick or a long wooden ruler, a protractor and a compass. Find an empty space such as a park or a parking lot where there are few tall buildings, trees or hills to obstruct the sun. Find your location's latitude. Subtract this number from 90. This will be the angle you will affix the stick in the ground. If you are in the Northern Hemisphere, use your compass to find south and point the stick in that direction. If you're in the Southern Hemisphere, point the stick or the ruler to the north. Using the protractor to fix the stick in the ground at the angle you just calculated — remember to point it in the direction opposite to the hemisphere you are in. Wait until noon and see the shadow of the stick disappear. At noon, the stick will have no shadow at all!

4. It determines when Easter is. The vernal equinox is important in Christianity because it sets the date for Easter Sunday. Easter falls on the first Sunday after the first full moon after the equinox.

The shamrock, often used to represent St. Patrick's Day, also is symbolic of the regenerative powers of nature. (Photo: SaigeYves/Shutterstock)

5. The vernal equinox symbol is a shamrock. The symbolic plant of the equinox in Druidry is the trefoil or shamrock, which is also associated with St. Patrick's Day. The shamrock is symbolic of the regenerative powers of nature, and spring is a time of rebirth, whether that means the resurrection of Jesus in Christianity or the rise of perennial flowers up from the cold ground.

6. It's not the only day when an egg will stand on its head. Wait, what? Apparently there's a long-held belief or rumor that you can stand an egg on its end on the first day of spring at the moment of the equinox. The theory is that some sort of gravitational magic happens due to the sun's equidistant position between the poles of the Earth. But it's nonsense, Snopes says, because you can stand an egg on its head any day of the year with the right egg plus a little trial and practice.

Why eggs? Snopes explains that, too:

Eggs — one of the most ubiquitous symbols of fertility and birth — have long been associated with the beginning of spring, and hence with the equinox. Many, many superstitions involving the breaking, balancing, burying, decorating, reading (for purposes of divination) and hiding of eggs have come to be part of the annual spring celebration. (The linking of egg-balancing with spring celebrations is demonstrated by the fact that the practice is associated primarily with the vernal equinox and far less commonly with the autumnal equinox.)

On the vernal equinox, the sun rises directly over the central tower at Angkor Wat in Cambodia. (Photo: សុខគឹមហេង/Wikimedia Commons

7. Some famous historical monuments honor the equinox. At Stonehenge in Wiltshire, England, the vernal equinox sun rises precisely between two stones. Early Egyptians built the Great Sphinx so that it points directly toward the rising sun on the spring equinox. At Angkor Wat, a temple in Cambodia built between 1113 and 1150 AD, the sun rises directly over the top of the central tower on the vernal equinox. And at Chichén Itzá in Mexico, the sun of the late afternoon creates the illusion of a snake on the northern staircase.

8. Easter isn't the only start-of-spring celebration. The first day of spring also marks the beginning of the 13-day celebration of the Persian New Year, which is called Nowruz. Also the colorful springtime festival of Holi is celebrated in India and other Southern Asia countries around the time of the vernal equinox.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.